BoilerFaultCodesViessmann 18 Fault Code
The Viessmann 18 fault code means: your boiler has lost its connection to the outside temperature sensor, meaning it cannot automatically adjust your heating based on the weather.. It appears on 9 Viessmann models in our database.
Technical description: Controls according to 0°C outside temperature: Outside temperature sensor lead break
What causes the Viessmann 18 fault?
This fault occurs when the boiler's control board loses the electrical signal from the sensor located on the outside of your home. The most common reason is a broken or corroded wire between the sensor and the boiler, often caused by exposure to the elements or accidental damage. In some cases, the internal components of the sensor itself may have failed due to age.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the 18 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 18 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026